Changing from Wind to Solar Energy

A correctly installed solar panel is an essential component of a cruising boat's energy supply system

A correctly installed solar panel is an essential component of a cruising boat’s energy supply system

When I moved my new Nicholson 32 sloop, Alibi of Bridham, from a marina to a mooring this summer I also had to rethink my power requirements, since the change meant severing my umbilical to the grid. Although I had a powerful (read: noisy) wind generator as an alternative power source, along with a small photovoltaic (PV) solar array to keep the engine’s cranking battery topped up when the wind dies, I’ve since decided to lose the noisy windmill and go wholly solar.

An important part of this process has been doing my best to save energy to help balance the budget. With this in mind, I’ve installed LED bulbs both belowdecks and in my nav lighting. The resulting 90-percent reduction in lighting consumption has allowed me to remove one of the four 110Ah batteries from my domestic bank, leaving less to charge and shedding excess weight.

I also ditched my 2kW inverter (and associated AC appliances) for a 500W model, fitted a water-cooled refrigerator condenser with a holding plate, and installed a low-power digital radar. As a result, I trimmed my total consumption by about 40 percent. In fact, if things turn out okay, I might even someday remove another battery.

How long a piece of string?

My previous boat had a 100Ah cranking battery and four 110Ah lead-acid house batteries. Two 15W PV panels put roughly 4 to 5Ah into the former on a clear summer’s day and around 2Ah per day in the winter. My trusty Ampair 100 wind generator provided around 25Ah per day (I usually switched it off at night when cruising) under normal conditions, which when combined with the output from the solar panels enabled us to spin the engine for charging only every third day when at anchor.

The author’s new walk-on Solara panel—note the waterproof cable gland

The author’s new walk-on Solara panel—note the waterproof cable gland

Considering the 40 percent projected energy savings on my present boat, I reckoned 60 to 70W of solar power should keep both battery banks almost topped up. It won’t be quite enough for all my power needs, especially on an extended cruise, but given the free deck space I have aboard Alibi of Bridham, it’s the best I can do without building a cockpit arch or buying a catamaran. In a pinch, I also have a portable 32W Spectraflex PV panel that I can deploy at anchor.

With a maximum theoretical output of around 6 amps, I’ve installed a fairly basic, two-stage Phocos 10A charge controller. I already have a Voltage Sensitive Relay (or VSR, also known as an ACR) on my boat that feeds the cranking battery until it reaches a pre-set voltage (usually 13.8V) before sending power to the house bank, so there is no need for a dual-bank model. I simply feed the panel’s output to the cranking battery via the controller, and the VSR does the rest.

I chose to install a single 68W Solara semi-flexible panel because the 50W model performed very well in a gear review I carried out last year. It also looks like it can survive a fair bit of mistreatment. My calculations, however, may prove disastrously incorrect if we have another overcast and rainy summer.

Installing a solar panel is a simple task, well within the capabilities of most boat owners

Installing a solar panel is a simple task, well within the capabilities of most boat owners. Art by Alastair Garrod


Monocrystalline, polycrystalline or amorphous cells?

The active ingredient in almost every PV cell is silicon, which can be produced from high purity silica, or silicon dioxide, commonly know as sand. Silica can be melted in the presence of carbon in an electric arc furnace that has carbon electrodes. A reaction produces liquid silicon that can be drawn off from the bottom of the furnace and cooled before being processed into electronic grader crystals.

The difference between monocrystalline and polycrystalline solar cells is simply that the former are produced from a single crystal of silicon, thinly “sliced” to make a cell that is as pure as possible. The latter are constructed from numerous smaller crystals that are less pure and less efficient. As a result, polycrystalline cells provide slightly less power for the same area of silicon, usually around 11 to 12.5W/ft², compared to 13 to 15.75W/ft².

On the positive side, polycrystalline cells are simpler to produce and a little cheaper. Amorphous thin-film panels are only about half as efficient as crystalline panels and are usually used in small electronic appliances such as calculators and watches. Despite this lack of efficiency, the power output from amorphous panels doesn’t deteriorate in extreme heat, as is the case with solid crystal cells, and their makeup allows the panels to be flexible. Amorphous panels are typically encapsulated in durable, flexible plastic and can be stepped on without being damaged.

Rigid or flexible?

Properly marinized PV panels can be walked on or have things dropped onto them. Aluminum-framed, glass-covered domestic PV panels are considerably cheaper, but can only be mounted in a well-protected area, such as across the top of a cockpit arch or on a rigid bimini. They are also noticeably heavier than flexible panels and require fairly solid support and mounts.

Flexible and semi-flexible panels can also be rolled away when not needed and can conform to the contour of a sloping cabintop. They can be laid out where and when you need them, as well as easily moved to face the sun.

How much power will I get?
It’s important to be realistic about how much power you can get from a solar panel. Yes, a 24W/12V PV panel will, in theory, produce around 2 amps of “peak power” charging current when in strong direct sunlight, but this rarely happens in practice.

If your panels are fixed to the deck, the movement of the sun will considerably reduce their effectiveness. Taking the seasons into account, as well as the possibility that the panels will be shaded or partly obscured at times, your panels will produce a lot less power than their peak ratings indicate. As a general rule, the average annual output is likely to be around 30 to 50 percent of the stated rating.

At the least, a small PWM or shunting regulator should be fitted to control charging (left); An MPPT controller is more efficient at bringing batteries to full charge (right)

At the least, a small PWM or shunting regulator should be fitted to control charging (left); An MPPT controller is more efficient at bringing batteries to full charge (right)


A PV panel putting out less than 10W, if connected to a reasonably sized deep-cycle battery, is unlikely to need a charge controller/regulator, provided the panel is diode-protected to stop it from discharging current at night. When using more powerful panels, a controller is usually required.

A controller provides several levels of protection to a solar circuit, such as overcharge prevention, temperature compensation and battery-type regulation. It also halts any reverse discharge and provides accidental short-circuit or reverse-polarity protection. Some more expensive units have a remote display option, or at least a row of LEDs to indicate how much voltage and current the panel is producing.

Modern controllers often incorporate Pulse Width Modulation (PWM) technology, which boosts the power going into your batteries, helping to overcome any internal resistance. This revitalizes batteries that have become sulfated due to repeated partial charging. An MPPT (Multiple Point Power Tracking) controller with a two-stage bulk/float regimen makes charging more efficient still. It’s also possible to buy a dual-bank voltage controller with a built-in VSR circuit, like the one I have on Alibi of Bridham.

No matter what model you choose, be sure to get the rating right—a 100W/12V solar panel will be pumping out just over 8 amps at between 18 to 24V unregulated and can overcharge your batteries. It’s best to choose a controller with some spare capacity, especially if there’s a chance you may add another panel later.

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Blocking diodes
Blocking diodes are usually fitted to the panels by the manufacturer to ensure the correct directional flow and to ensure that batteries to which they are connected don’t discharge back into the panel at night. In some cases, you may need to install an additional diode between interlinked panels to stop them adversely affecting each other.

On good-quality panels, there are also diodes between each cell to prevent the shading of one or more cells from drastically reducing overall output. This is an important feature on a boat, as there are usually things shading the panels, especially if they’re deck-mounted.

Installation and wiring
My old boat had two 15W PV panels deployed at acute angles to the sky, diagonally opposed, rather like a skylight. One or the other was always pointing at the sun, but when the sun was rising or setting one panel was always in the shade. On Alibi of Bridham I decided to mount my new panel flat onto the deck instead.

In most cases, semi-flexible panels are ideal for this kind of installation. They typically have a hole in each corner, so you can screw them to a deck. To avoid making holes, you can also glue them down with a good polyurethane adhesive, such as Sikaflex 292i or Marineflex fast cure. On Alibi of Bridham I used several large jerries of water to hold the panel down while the adhesive set.

Leading the cables below can be tricky, especially if they’re short. Rather than drill through the cabintop, I led my wiring through the hatch garage via a deck gland. The cable was then run by the most direct route possible to the charge controller and then to the batteries.

Most bluewater cruising yachts don’t rely entirely on PV panels or any other alternative power source. As on my boat, the primary use for solar panels is to keep the engine cranking battery charged at all times. This way you can always use the engine’s alternator to fully recharge the domestic battery bank. This approach is particularly effective if you have a smart charge regulator fitted to your alternator, as it will charge batteries far more quickly than any alternative power source.

Whether my single PV panel will allow me to only run my engine for two hours every third day when cruising remains to be seen. I’ll let you know how I got on after my summer cruise.


Bruce Schwab Energy Systems,

Coastal Cool-Aids,

Coastal Solar Sales,

Custom Marine Products,





West Marine,

Photos by Peter Nielsen, Duncan Kent, Coastal Cool Aids, and Coastal Solar Sales

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